#include <cstdio>
#include <algorithm>
using namespace std;
const int MAXN=3010;
double c[MAXN];
int n;
double a0,an1;
int main(void){
    while(~scanf("%d",&n)){
        scanf("%lf",&a0);
        scanf("%lf",&an1);
        for(int i=1;i<=n;i++){
            scanf("%lf",&c[i]);
        }
        double ans=n*a0+an1;
        for(int i=0;i<n;i++){
            ans-=2*(n-i)*c[i+1];
        }
        printf("%.2lf\n",ans/(n+1));
    }
    return 0;
}
